Republicanism was an ideology based on the concept of government chosen by the people, and centred loosely on the state of the eponymous Roman Republic. It was inflamed by the rhetoric of humanist philosophers and dominated the late 18th and early 19th century. However, the concept of the "social contract" and "popular sovereignty" so eloquently phrased by those men seems to have worked much less well in practice than on paper. The abuses committed by republican experiments and the consistently bloody results of its implementation led eventually to the fall of all states that espoused it, and it was posthumously regarded as unnatural, flying in the face of all established reason. Many have also pointed out that the Roman Republic which the Republicans so admired was really just an oligarchy, and that it naturally became more autocratic as the empire grew.
Some today, however, regard the "young god that failed" as having been judged too harshly. They believe that its implementation led to disaster only by the lack of strong checks on the power of magistrates and restrictions on the natural tendency towards mob rule.
In this day and age of increased freedom of expression, the ideas of those old philosophers have seen new traction. But can the greatest and most ambitious of the enlightenment's children have ever come to succeed or replace monarchy entirely as a mode of governance?
Note that this is only a hypothetical question. It does not reflect a treasonous viewpoint that His Most Catholic Majesty does not possess a right to the state, nor does it attempt to defame, sully, or otherwise injure the dignity of his most virtuous and high blooded majesty.